1. Field of the Invention
The present invention relates to network devices, and more particularly, to a transmitter of the network device and a method thereof.
2. Description of the Prior Art
Network technology is rapidly developed in recent years, and the demand for bandwidth is also growing increasingly. Along with this, the rate of network devices is upgraded from 10/100 Mbps to 1 Gbps. For example, each port of a 1 Gbps Ethernet device has four channels, each of which includes a transmitter and a receiver for transmitting and receiving differential transmission signals respectively, thereby communicating with other network devices.
FIG. 1 is a diagram of the transmitter of a conventional Ethernet device. As shown in FIG. 1, the transmitter 10 lies within an IC chip, and includes a voltage-controlled current source 11, a differential amplifier 12, and feedback resistors Rf1, Rf2.
To avoid the overshoot of the differential transmission signal, the voltage-controlled current source 11 performs slew rate control. That is, when the level of the differential transmission signal changes, the rise time (or fall time) is controlled to prevent the signal level from changing too fast. However, when the Ethernet device operates in 1 Gbps transmission rate, the voltage-controlled current source 11 is required to output a differential current with 17 levels, so as to match with 17 levels of the differential transmission signal defined in the related Ethernet specification. Thus, the circuit within the voltage-controlled current source 11 would be very difficult to design and would consume much more chip space.